1. Field of the Invention
The invention relates to the field of semiconductor devices, and more particularly to integrated circuits used for precision measurements in automatic test equipment.
2. Description of Related Art
As part of the manufacturing process, semiconductor devices are subjected to various tests in order to identify faults. This testing can occur at multiple points in the manufacturing process, including testing done before packaging and testing done after packaging. Manufacturer testing of semiconductors is often performed using equipment referred to as automatic test equipment, or ATE. An ATE system can be used in a wide variety of applications, including the identification of defective semiconductors and the sampling of parts for quality control.
Automatic test equipment further includes specialized semiconductor devices known as precision measurement units, or PMUs, that are used to force a signal to a particular current or voltage, and/or to measure the voltage or current on a given signal. An example of a per-pin PMU device is the Edge4707 part manufactured by Semtech Corporation. This part is a four channel device in which each channel can be independently configured to force voltage or current and to sense voltage or current. ATE systems with a large number of individually controllable pins can be constructed using multiple PMUs and the PMUs can have multiple ranges of operation. In the case of the Edge4707, there are four current ranges available in the force current mode, with each being selectable using an input selection control and external resistors. Specifically, a range of xc2x12 xcexcA, xc2x120 xcexcA, xc2x1200 xcexcA or xc2x12 mA can be selected using a two-bit control input, and the range is enabled through the use of four external resistors, nominally 1 M, 100K, 10K and 1K ohms, respectively. In the force voltage mode, an output voltage of xe2x88x922V through +13V can be selected.
A drawback of conventional PMUs is that when the output pin is disconnected from the device under test, there no longer exists a feedback path from the output amplifier to the pin and back to the input of the amplifier. This causes the output amplifier to drive the output to the maximum value possible, i.e., either maximum positive or maximum negative depending on noise and other circuit conditions. When the output pin is reconnected to a device under test, a momentary voltage or current spike would be present on the output pin that could damage the device under test. One way to avoid the current spike is to power down the entire PMU between each use of the part. But, this adds time to the semiconductor test procedure and additional complexity to the overall design of the ATE.
It would therefore be desirable to provide a high-impedance mode for the output pin of a PMU, so that when a device is not being tested, the output would be in a disconnected state. Such a high-impedance mode would be utilized in ATE between the testing of devices. This would prevent the undesirable current spikes from occurring and avoid the need to power off the entire chip between operations.
The present invention provides a high-impedance mode for an output of a precision measurement unit (PMU). The PMU includes an output amplifier that provides a forcing voltage or current to a device under test. When the high-impedance mode is activated, the output amplifier is decoupled from an output terminal of the PMU and the output amplifier is disabled. This prevents the voltage on the output terminal from rising in an uncontrolled manner, and prevents current spikes from the output terminal when connected to a device under test. The high-impedance mode is deactivated to permit connection of the PMU to another device under test by re-coupling the output amplifier to the output terminal and enabling operation of the output amplifier.
In one embodiment of the invention, the output of the output amplifier is tied to ground during the time that the output is in the high-impedance mode. When the output is re-connected, the output amplifier is no longer tied to ground.
In another embodiment of the invention, an internal feedback path is established during the time that the output is in the high-impedance mode. When the output is re-connected, the internal feedback path is disconnected.
A more complete understanding of the method and apparatus for providing a high impedance mode for an output of a precision measurement unit will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.